The WHO has proposed that using treatment as prevention (TasP) could play a key role in eliminating HIV; i.e., reduce incidence to less than one new infection per 1,000 people per year. Substantial evidence shows treatment can be highly effective in protecting an individual against HIV infection. However, for TasP to be an effective public health intervention, and possible tool for HIV elimination, it is essential for treatment to also have a population-level effect and reduce incidence; while such an effect has been hypothesized, it has not yet been empirically demonstrated. The overarching goal of this proposal is to determine whether TasP could eliminate HIV. We propose to first determine whether, in a high-risk community in a resource-rich country, a high level of treatment as therapy has in fact reduced incidence as hypothesized, and also decreased the size of the hidden epidemic (i.e., the number of undiagnosed individuals), a variable that is crucial to the success of TasP. We will make these determinations by analyzing an exceptional ~30 year dataset to estimate past and present incidence in the MSM community in Denmark, where treatment coverage is high and viral suppression is the highest worldwide. By providing an estimate of present conditions in this community the results from our analysis set up the second part of the work we propose: predicting whether TasP can eliminate HIV in this community and, if so, how long it would take. Our results will be particularly informative for policymakers in Denmark. But they will also have more general significance by providing important insights into the feasibility of using TasP to eliminate HIV in more challenging settings. Taken together, our results will inform health policy makers on the conditions that need to be met, and the approach that needs to be taken, for TasP to have a significant impact on reducing HIV epidemics in a major risk group in resource-rich countries. Specific Aims: Aim 1: Determine whether widespread treatment has been associated with decreased incidence and reduced the hidden epidemic. We will use a Bayesian CD4-staged back-calculation approach, and diagnosis data from a national longitudinal study to reconstruct the HIV incidence curve (and hidden epidemic) in the Danish MSM community from the mid-1980's onwards. We will then use treatment data from 1994 onwards, in combination with estimates from the back-calculation, to determine whether incidence and the hidden epidemic have decreased; and are associated with treatment. Aim 2: Determine whether TasP-based strategies could eliminate HIV within 10 years. We will use modeling to identify multiple TasP-based strategies that could lead to elimination in the Danish MSM community, predict elimination times and identify key components of TasP-based strategies that minimize the elimination time. To achieve these goals we will model transmission dynamics, construct response hypersurfaces and build Classification and Regression Trees.